The present invention relates to packaging materials particularly usable in a high temperature atmosphere or environment such as in a state heated by a microwave oven (electronic oven) and also relates to packaging bags or containers formed of such packaging materials capable of automatically reducing inner pressure of the packaging bags and containers and preventing them from being broken or burst when they are heated by the microwave oven or like in contents filling-up condition.
Usually, frozen food and chilled food are frozen in a state contained and hermetically sealed in a packaging bag manufactured by a packaging material of plastic and a container having a cover member, covering a body (tray) of the container, which is formed of such packaging material. Further, it is to be noted that such packaging bags and containers may be called hereinlater totally as packaging bag or like for the sake of convenience. Such foods packaged are then conveyed and stored in the frozen state, and when used, the frozen food or chilled food is heated and cooked by means of microwave oven through a heating process. However, at a time when such packaged food is heated in the microwave oven, there is a fear of increasing an inner pressure of the sealed bag or package due to the expansion of inner air or steam generated from the contained food, breaking or bursting the bag or package with a large bursting voice and splashing the contained food, being dangerous and providing an inconvenient problem.
Incidentally, in a box lunch, daily dish, frozen food, chilled food and like, which are called as xe2x80x9cbox lunchxe2x80x9d totally, herein, piece of dried bonito, fish flour, pepper, green laver, powdered cheese, mayonnaise, or like, which will be totally called as seasoning, is usually appended or added when the box lunch is sold in convenience stores, supermarkets or like. Such lunch box is heated and cooked by the microwave oven, for example, when eaten, and in such time, when the seasonings which are packaged in small packaging bags or like are heated together with the box lunch, there is a fear of swelling or expanding an air, steam or like in the small packaging bags, increasing an inner pressure of the bag and then breaking or bursting the bags with large bursting noise, the seasonings being splashed or scattered.
In order to obviate such problem, in the prior art, only the box lunch is heated by the microwave oven, for example, after taking out the small bag, pack or package, or the small bag is pierced before the heating to prevent the increasing an inner pressure of the packaging bag. However, it is inconvenient to previously take out the small bag or package from the box lunch or to pierce the same, also providing a troublesome problem and being inconvenient.
In order to obviate such inconveniences or problems, in the prior art, Japanese Patent Laid-open Publication No. HEI 9-40030 discloses a packaging container in which perforations as through holes for degassing or venting an inner gas in a plastic container are formed along a peeling (releasing) agent applied along a seal surface of the container due to the increasing of the inner pressure at the time of heating the container. Furthermore, Japanese Patent Laid-open Publication No. HEI 9-272180 discloses a package container in which such perforations as those in the above publication are formed on the seal surface along the peeling (releasing) agent applied between a heat-resistant thermoplastic resin layer and a sealant film. Such perforations are formed (occur) through the peeling of the sealed portion which has been sealed through the peeling agent layer by the increasing of the inner pressure in the container due to the heating in the microwave oven, for example. That is, through the peeling or releasing of the peeling agent layer of the sealed layer portion, the strength of the sealed portion is weakened, and as a result, the sealed portion is peeled along the released portion, and accordingly, the perforations (through holes) are formed to the container.
Furthermore, another prior art discloses a use of unwoven cloth as packaging material for preventing the package made of the unwoven cloth material from being burst such as disclosed in Japanese Patent Laid-open Publication No. HEI 9-249267, or further prior art discloses a formation of an unsealed portion for suppressing an increasing of an inner pressure of the container such as disclosed in Japanese Patent Laid-open Publication No. HEI 11-178713. Further, for example, Japanese Patent Laid-open Publication No. HEI 5-49462 discloses previous formation of small holes. All such prior art references teach the technique for previously forming the degassing holes to prevent the packages, bags or packs from being burst without providing a sealed condition of the package or container.
However, since the peeling (releasing) agent layers mentioned above are formed essentially to be easily peelable, there is a fear that such peeling agent layers are easily peeled by a pressure or impact applied in a usual transporting or storing time. Moreover, in the case of sealing the container or packaging bag by coating the peeling agent to the inner surface thereof, there is a fear of the peeling agent being contacted to the inner content by being easily bled from the sealed portion, thus being inconvenient. Because of such reasons, in the conventional art, it is necessary to take care of suitably forming the pattern shape of the peeling agent layer or to form the same so as not to be bled from the sealed portion. Therefore, there are provided some complicated matters in manufacturing processes or like.
Further, in the structure of forming the degassing holes, since the package or container is not in a sealed state, there is a fear that the inner content of the container or the seasonings in the small pack or package may be deteriorated through the contact to oxygen, steam or like during the transporting, selling or like process, which provides a significant problem.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a packaging material particularly suitable for a packaging bag or a container usable in an atmosphere heated by, for example, a microwave oven and also provide a packaging bag or container formed of such packaging material, which is capable of being easily treated to be manufactured, being not peelable during usual transporting or storing time by any pressure or impact applied thereto and being heated safely in the microwave oven or like with an inner content being packaged therein.
This and other object can be achieved according to the present invention by providing, in a first aspect, a packaging material comprising:
a heat resistant base layer;
a sealant layer; and
a resin layer disposed at least a portion between the heat resistant base layer and the sealant layer,
the resin layer being composed of a resin material having a predetermined strength in a non-heated state of less than a room temperature and the strength is lowered in a heated state.
Further, according to a second aspect of the present invention, there is provided a packaging bag formed of a packaging material having a predetermined portion to be sealed, the packaging material comprising a sealant layer facing inside when a packaging bag is formed, a heat-resistant base layer and a resin layer disposed at least a portion between the heat-resistant base layer and the sealant layer, the resin layer being composed of a resin material having a predetermined strength in a non-heated state of less than a room temperature and the strength is lowered in a heated state, wherein the resin layer is formed so that the sealed portion has at least a portion which transverses from an inside portion towards an outside portion of the packaging bag.
Furthermore, according to a third aspect of the present invention, there is also provided a container having a container body and a container lid to be applied to the container body and formed of a packaging material and having a predetermined portion to be sealed, wherein the packaging material comprises a sealant layer disposed so as to face the container body when the container lid is applied to the container body, a heat-resistant base layer and a resin layer disposed at least a portion between the heat resistant base layer and the sealant layer, the resin layer being composed of a resin material having a predetermined strength in a non-heated state of less than a room temperature and the strength is lowered in a heated state, wherein the resin layer is formed so that the sealed portion has at least a portion which transverses from an inside portion towards an outside portion of the packaging bag.
According to the various aspect of the present invention of the characters mentioned above, when a packaging bag or container formed as food packaging bag or food containing container by using the packaging material of the present invention in the above first aspect and when such packaging bag or container is heated, portions of the resin layer and the sealant layer are broken because the strength of the resin layer of the packaging material is lowered when heated. As a result, the inner pressure of the packaging bag or container is reduced without splashing the inner content through the breaking, and furthermore, the resin layer disposed between the heat-resistant layer and the sealant layer does not directly contact the inner content. Accordingly, in a case where the resin layer is formed in pattern, it is not necessary to so severely control the pattern forming performance and the pattern can be easily formed, preferably, on the entire surface thereof
Furthermore, according to the packaging bag and the container of the second and third aspects of the present invention, it is necessary for the seal portion of the packaging bag and the container formed of the packaging material of the first aspect to have the resin layer so as to transverse (intersect) the seal portion. This seal portion is broken or burst when heated and the inner pressure thereof is reduced. Different from conventional packaging bag or container, in those of the present invention, the forming position and the performance the resin layer are permitted at a proper extent, so that the packaging bag and the container can be easily manufactured. Furthermore, the resin layer effectively acting for reducing the inner pressure is disposed between the heat-resistant layer and the sealant layer so as not to directly contact the inner content, being advantageous. The sealing performance at a temperature less than the room temperature, such as in a frozen state is sufficient, so that the packaging bag and the container can be prevented form from being peeled or released from the seal portion during the transportation, maintenance or storage thereof, thus maintaining the preferred sealed condition.
In preferred embodiments of the above first to third aspects, it is preferred that the resin layer has a softening point in a range of 60xc2x0 C. to 90xc2x0 C. and is selected from one of ethylene-vinyl acetate group copolymer resin, polyamide-nitrocellulose-polyethylene wax group resin and resin composition including polyamide-nitrocellulose-polyethylene wax group resin.
In the above first and second aspect, it is preferred that a pair of the packaging materials are overlapped and heat-sealed so that the sealant layers face each other, a sealing strength of a region formed with the resin layer is to be more than 7 (N/15 mm) at a temperature less than a room temperature and to be less than 3 (N/15 mm) at a temperature more than 80xc2x0 C.
Furthermore, in the above first to third aspects, it is preferred that the sealant layer is a single-layered sealant layer or multi-layered sealant layer selected from groups consisting of low-density polyethylene film, ultra-low-density polyethylene film, straight-chained low-density polyethylene film, intermediate-density polyethylene film, high-density polyethylene film, non-oriented polypropylene film, ethylene-vinyl acetate copolymer film, ethylene-acrylic acid copolymer film, ethylene-methacrylic acid copolymer film, ethylene-methylacrylate copolymer film, ethylene-ethylacrylate copolymer film, ethylene-methylmethacrylate copolymer film, and ionomer film.
It is also preferred in the above first to third aspects that heat-resistant base layer comprises one selected from groups consisting of oriented polyethylene terephthalate film, silica-deposited polyethylene terephthalate film, alumina-deposited polyethylene terephthalate film, oriented nylon film, silica-deposited oriented nylon film, alumina-deposited oriented nylon film, oriented polypropylene film, polyvinyl alcohol-coated oriented polypropylene film, nylon 6/methaxylene diamine nylon 6 co-extrusion co-oriented film, and polypropylene/ethylene-vinyl alcohol copolymer co-extrusion co-oriented film.
Still furthermore, according to the above three aspects, it is particularly preferred that the heated state is caused through a heating of a microwave oven. That is, the packaging material, packaging bag and the container of the characters mentioned above are particularly usable for or served to the microwave oven by which to be heated.
In this particular aspect, the packaging bag is a small bag containing a seasoning appended to food to be heated by the microwave oven. The seasoning material is a non-fluid material such as a piece of dried bonito.
In the second aspect, i.e. packaging bag, of the present invention, the transversing portion of the resin layer is formed with an easily openable portion, which may comprises a projecting seal portion formed so as to project inside the packaging bag. The projecting seal portion is formed with a hole or cutout for shortening a transversing length of the resin layer to a portion of at least the packaging material having the resin layer. The easily openable portion is a portion formed to the heat-resistant base layer, in the thickness direction thereof, as cutout, half-cut, score line, emboss or like.
According to the preferred embodiments of the packaging material, packaging bag and container of the present invention, when the small packages of seasoning such as dried bonito is heated together with the box lunch by the microwave oven (electronic oven), the strength of the resin layer is lowered, and the resin layer and the sealant layer are partially broken and then opened. As a result, the inner pressure of the bag or container is reduced without scattering the seasoning in the small package or like. Further, such package has a sufficient sealing strength in an environment having a temperature less than a room temperature, i.e., in a usual environmental condition in transportation, maintenance, storage or like time thereof, so that the sealed condition can be maintained in such usual state (non-heated state), thus maintaining the quality of the inner content.
Still furthermore, the easily openable portion is formed to the resin layer transversing portion and the inner pressure in the bag or container increased in accordance with the heating of the microwave oven is concentrated on this easily openable portion, so that this easily openable portion can be selectively easily broken to be opened. As a result, the packaging bag or container is opened before the inner pressure is excessively increased, so that the pressure of steam generated through the heating from the inner content can be weakened and, hence, any bursting noise can be reduced at the opening time, as well as the swelling of the packaging bag or container can be prevented, thus being safely utilized by a user.